The subject matter of this invention relates generally to motor protection devices and more specifically to microprocessor controlled protective relays of the kind that are utilized to control the number of motor starts in a fixed time interval.
Microprocessor controlled motor protector devices where starts per unit of time limiters are known. An example can be found in U.S. Pat. No. 4,434,390 issued Feb. 28, 1984 to R. T. Elms and entitled "Motor Control Apparatus with Parallel Input, Serial Output Serial Conditioning Means". The aforementioned patent is assigned to the assignee of the present invention. Another example can be found in a user's manual entitled "IQ-2000.TM. Model B Motor Command System User's Manual" TD-11-720-B provided by the Control Division of the Westinghouse Electric Corporation. It is known to provide motor detector devices with the capability of preventing more than a maximum programmable number of starts per programmable unit of time. This is often necessary because each time a motor is started, the amount of in rush current which is usually significantly higher than the full rated current of the motor adds a an accumulated component of heat to the motor which can only be dissipated with time. Consequently, repeated starting of the motor causes an accumulation of heat which can be detrimental to the safe operation of the motor. In the past, a microprocessor based motor control apparatus was provided which used a preprogrammable fixed time against which all of the starts were compared. For example, if it was determined to allow only five starts in four hours as soon as five starts occurred within a four hour time period, the motor protected apparatus prevented any subsequent start. However, it is known that after the passage of a certain amount of time, it would be allowable once again to have another start. In a trivial case, it can easily be seen that once four hours had elapsed from the last start that five more starts could easily be accommodated in another four hour period. It would be desirous therefore if motor protective apparatus which had a starts per interval of time characteristic could be provided which measured the fixed preprogrammed starting time from the last start rather than on a fixed time basis. This would improve the overall efficiency of the motor and allow the truly maximum number of starts to occur in any given interval of time.